Thermal cells for body warmers are well known in the art, particularly those which use a reactive iron powder and activated carbon mixture. Such cells require oxygen to produce a controlled exothermic reaction which provides heat over several hours. Typically, the mixture is packaged in an air permeable pocket, which is sealed in an impermeable outer pouch until ready for use. The permeable pocket may be a nonwoven material. It is difficult to provide in a nonwoven material a diffusive oxygen permeability at 0.21 atmosphere driving force ranging from about 0.5.times.10.sup.5 cubic centimeters/100 square inches/day to about quadruple that permeability. It is difficult because greater than about 95% but less than 100% of the nonwoven's surface must be occluded with an impermeable material in order to do so. This is a permeability range that others have avoided. Liquid and air filtration processes require much higher permeability ranges (typically about 50% open area), and gas separation membranes have lower permeability ranges (zero percent open area).
Gas permeable membranes have been available for separation processes. For example, U.S. Pat. No. 5,102.552 to Callahan et al., issued Apr. 7, 1992, discloses a UV curable polymer coated onto a microporous support having an average pore size from about 0.005 microns to about 0.2 microns. U.S. Pat. No. 3,754,375 to Bouchilloux et al., issued Aug. 28, 1993, discloses an "anisotropic membrane having excellent mechanical properties combined with good permeation characteristics. It comprises a vinyltriorganosilane polymer or copolymer" having a dense layer of 0.01 to 10 micron average thickness and a porous layer 20% to 80% open area.
Others have even applied gas permeable membranes to heat cells, but with limited success. For example, U.S. Pat. No. 5,046,479 to Usui, issued Sep. 10, 1991, discloses a method of controlling oxygen permeation through a microporous film which is subject to a "heat fusion treatment" intended to restrict oxygen permeability to a disposable body warmer. A flat bag containing iron powder heat generating agent has an air permeable surface having an air permeability per unit of 5000 to 10,000 sec/100 cc. Such microporous films, unfortunately, are very expensive.
Others have poked small holes in an impermeable film with needles to provide oxygen permeation for heat cells in the desired range. This process is practically limited to a hole pattern wherein holes are relatively few and large. Because of this limitation, heat cell size is necessarily large. Large thermal cells have the disadvantage of being inflexible to bending to the contours of the body surface to which they are applied. Smaller "pores" created by a process that generates a finer pattern having the same permeability enables many smaller cells to be formed having greater surface conformability. Also, smaller holes are better at preventing granular thermal cell chemistry from falling out. In processing a film to provide permeability with needles, slight changes to permeability cannot easily be made. Also, film tension in the process tends to tear or otherwise enlarge holes when needle piercing occurs; thus, hole size may not be easily controlled when web tension varies.
Others have smeared adhesive onto porous webs. An example is U.S. Pat. No. 5,558,344 issued Nov. 26, 1996 to Ahr et al., which teaches the use of a printing roll having peripheral cells to apply a resinous material to a substrate web. The printing roll has at least 100% greater surface speed than that of the substrate web. The wiping process created by the surface speed differential causes the resinous material to penetrate into the substrate to cause the substrate to become water impermeable. No attempt is made by Ahr et al. to provide less than 100% coverage and there is no suggestion of using such a process for accurately controlling oxygen permeability of the substrate within a narrow range.
It is an object of the present invention to provide an inexpensive method of making an oxygen permeable material, which has a permeability in the narrow range needed for controlling heat generated within thermal cells.
It is a further object of the present invention to provide a method for making a gas permeable material, which is directly sealable to the flange of an impermeable pocket containing a thermal mixture without the need for heat sealing.
It is yet another object to provide a method for making a gas permeable material wherein a simple process change can be made to slightly vary the oxygen permeation of the material within a desired narrow range.